Since grain oriented electrical steel sheets are mainly used as an iron core material for transformers and motors, it is strongly demanded to be excellent in the magnetic properties, particularly excellent in iron loss property (low in iron loss). To improve the iron loss property, it is effective to highly align secondary recrystallized grains in a steel sheet product into {110}<001> orientation (so-called Goss orientation) or to reduce impurities.
However, there is a limit in the effect of improving iron loss by the abovementioned crystal orientation control or impurity reduction. To this end, there has been developed a method of reducing the iron loss by introducing non-uniformity into the steel sheet surface through a physical means to subdivide a magnetic domain width, or so-called “magnetic domain refining” method. For example, JP S57-002252 B proposes a technique wherein a laser is irradiated to a final product sheet to introduce a high dislocation density region into a surface layer of the steel sheet to thereby narrow a magnetic domain width and reduce an iron loss. JP H06-072266 B proposes a technique of controlling a magnetic domain width by electron beam irradiation.
When the laser irradiation or electron beam irradiation (hereinafter referred to as “beam irradiation” simply, and an irradiation apparatus therefor is referred to as “beam irradiation apparatus”) is performed onto the steel sheet surface, it is difficult to irradiate the full width of the steel sheet with a single beam irradiation apparatus due to restriction on a convergence or an irradiation rate of beam, i.e. a rate of scanning beam onto the steel sheet surface (hereinafter referred to as “scanning rate” simply). In such a case, irradiation is performed by disposing a plurality of beam irradiation apparatuses in a widthwise direction of the sheet so that a displacement is necessarily caused in a “juncture” between regions covered by the individual beam irradiation apparatuses (hereinafter referred to as “beam-irradiated region”). Two kinds of discontinuities (hereinafter also referred to as “displacement”) exist in the juncture, one of which is a displacement in the rolling direction. Such a displacement is a so-called “incoherent” phenomenon that when two beam irradiation apparatuses are not controlled in synchronization, two irradiated regions are shifted to each other in the longitudinal direction of the steel sheet (rolling direction: RD direction). Another displacement is a displacement in a widthwise direction of the sheet (TD direction). Moreover, the displacement in the widthwise direction of the sheet may have two adjacent beam-irradiated regions overlapping in the widthwise direction and the two regions being separated without overlapping.
To minimize the discontinuities of the irradiated regions in the longitudinal and widthwise directions, or the quantity of the displacement, it is necessary to properly set the irradiated region of the each beam irradiation apparatus and simultaneously control the adjacent beam irradiation apparatuses in synchronization. Even if the setting at an initial state goes smoothly, meandering of the steel sheet is caused during continuous irradiation of the steel sheet or an error is caused in the beam-irradiated region due to the aging degradation of an optical system, whereby the displacement may be caused in the beam-irradiated region at the juncture in the longitudinal direction and/or the widthwise direction to render the beam-irradiated regions into the discontinuity. If such a discontinuity exists in the steel sheet, deterioration of the iron loss property is caused.
As a method of resolving such a discontinuity, for example, JP H06-116654 A discloses a beam irradiation method wherein a meandering amount of a steel strip is detected to change a scanning range of beam irradiation. Also, there is a method of sensing the beam-irradiated region in some way to feedback control the beam-irradiated region.
In the conventional method of JP H06-116654 A, however, an additional installation is required or the scanning rate is delayed associated therewith so that productivity is largely blocked.
It could therefore be helpful to provide a grain oriented electrical steel sheet having an excellent iron loss property and a method of producing the steel sheet in a good productivity.